Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system may allow the vapors to be purged into an engine intake manifold for use as fuel.
The purging of fuel vapors from the fuel vapor canister may involve opening a canister purge valve coupled to a conduit between the fuel vapor canister and the intake manifold. During a purge operation, vacuum or negative pressure in the intake manifold may draw air through the fuel vapor canister enabling desorption of fuel vapors from the canister. These desorbed fuel vapors may flow through the canister purge valve into the intake manifold. As such, the canister purge valve may regulate the flow of fuel vapors (mixed with air) into the intake manifold via a sonic choke positioned between a valve in the canister purge valve and the intake manifold. Accordingly, the sonic choke may function as a flow restrictor in the purge path between the valve and the intake manifold. The sonic choke aids flow rate predictability when the intake manifold vacuum is deeper than 15 kPa. However, it also serves to limit the maximum possible flow rate. This restriction is particularly adverse at shallow intake manifold vacuums and during boost when the vacuum is sourced by an ejector (or aspirator).
In boosted engines, during boost conditions when the compressor is operative, the intake manifold may have a positive pressure. Herein, an aspirator coupled in a compressor bypass passage may generate vacuum that can be used to draw stored fuel vapors from the fuel vapor canister. However, purge flow rate through the aspirator may be lower because the sonic choke in the canister purge valve may excessively restrict canister purge flow to the suction port of the aspirator. Accordingly, purging of the fuel vapor canister via vacuum drawn from the aspirator may be severely diminished by the presence of the sonic choke in the flow path.
An example approach demonstrating an improved purging operation is shown by Stephani in DE 102011084539. Herein, an aspirator coupled in the compressor bypass passage directly communicates with the fuel vapor canister such that fuel vapors are purged to the aspirator from the fuel vapor canister without flowing through a canister purge valve. By directly coupling the fuel vapor canister to the aspirator, the metering effect of the sonic choke in the canister purge valve may be circumvented. A diverter valve in the compressor bypass passage regulates flow through the aspirator and therefore, purging of the fuel vapor canister.
The inventors herein have identified potential issues with the above approach. As an example, while the approach of Stephani may improve the purge flow rate at higher levels of boost, during conditions when the boost level is lower, such as at engine idling or vehicle cruising conditions, purge flow through the aspirator may be limited. During the same conditions, purge flow through the canister purge valve may also be limited due to the shallow level of intake manifold vacuum. Due to the reduced purge flow, the canister may not be sufficiently purged, degrading exhaust emissions, and rendering the engine emissions non-compliant.
The inventors herein have recognized that enhancing purge flow rate in the shallow intake manifold vacuum region is significant because the engine is often in this condition. Specifically, pumping losses are lower in the shallow intake manifold vacuum region of engine operation, making it a high engine efficiency region. Consequently, engine control systems may operate the engine in the shallow intake manifold vacuum region for a significant portion of a drive cycle. If the purging ability in the shallow manifold vacuum region is limited, a significant purging opportunity is lost. In addition, and serendipitously, high purge flow tolerance in the shallow manifold region is greater. This is due to engine airflow being relatively high in this region. As a result, if the canister is purged during shallow manifold vacuum conditions, the engine's fuel control system may be able to better tolerate the additional fuel and air in the purge flow. Consequently, to improve a canister purge flow rate during shallow manifold vacuum conditions, the inventors have developed a method for a boosted engine comprising: during a first condition, purging a canister via a purge valve; during a second condition, purging the canister via an ejector coupled in a compressor bypass; and during a third condition, purging the canister via a low restriction valve while bypassing each of the purge valve and the aspirator. In this way, canister purging is improved.
This approach adds flow rate enhancement without degrading the existing fine control at low air flow rates provided by the canister purge valve which includes a sonic choke. It provides a three-path purge solution (for deep intake manifold vacuum, shallow intake manifold vacuum, and vacuum via ejector at boost). The purge system enables air flow through the canister to be maximized during the defined drive cycle allotted to purge the canister. Maximizing the total air flow through the canister allows for the canister to be maximally emptied. For example, it may be possible to empty the canister to 80% empty over the drive cycle with an engine operating with reduced intake manifold vacuum. While the instantaneous purge flow rate may be limited during other purging conditions by other considerations such as the fraction of the engine's requisite fuel sourced via vapor purge, this may seldom be a limiting factor at boost or shallow intake manifold vacuum (a.k.a. manvac) conditions.
As an example, during conditions when there is sufficient intake manifold vacuum, a fuel vapor canister may be directly purged to an engine intake by opening a canister purge valve (CPV) and applying the intake manifold vacuum on the canister. During boosted engine operation, compressed air may be circulated through an ejector to the compressor inlet to generate vacuum at the ejector's neck. The generated vacuum is then applied on the canister by opening a bypass valve, allowing the canister to be purged to the compressor's inlet, while circumventing the more restrictive CPV. During conditions when the intake manifold vacuum is shallow, an alternate purge route may be used. Specifically, a special, low restriction purge valve may be coupled to the engine system in a branched purge line coupling the canister to the intake manifold. The special, low restriction purge valve may be a mechanical valve having a ball coupled to a spring-loaded valve, and optionally further including a sonic choke upstream of the ball to limit the maximum purge flow rate through the valve. As such, the sonic choke may not be needed if there is enough naturally-occurring restriction. However, if the valve was exceptionally free-flowing, the sonic choke would establish some restriction to improve controllability. However, this sonic choke is far less restrictive than the sonic choke contained in the classic CPV which serves the system during deep intake manifold vacuum. The low restriction valve may be configured to automatically (e.g., without electrical input) open at shallow manifold vacuum levels and automatically close at higher manifold vacuum levels. During engine operation in the shallow manifold vacuum region, when the low restriction purge valve is open, the bypass valve may also be commanded open so that fuel vapors can be flowed from the canister through the bypass valve and the low restriction purge valve into the intake manifold, while circumventing the classic, highly restricted canister purge valve. In some examples, based on engine boost levels when purging conditions are met (such as responsive to a tip-in or tip-out event), and corresponding intake manifold vacuum levels, a purge route may be selected and one or more of intake manifold vacuum or aspirator vacuum may be applied to a canister to enable a more complete cleaning.
In this way, the amount of fuel vapors that may be purged from a fuel vapor canister over an engine drive cycle may be increased, even as engine boost levels and intake manifold vacuum levels vary. The technical effect of coupling a less restrictive mechanical valve between a canister and an intake manifold is that shallow manifold vacuum levels can be advantageously used to more completely purge the canister, without necessitating flow through the more restrictive canister purge valve. By coupling the mechanical valve to the canister upstream of the canister purge valve (that also couples the canister to the intake manifold), a less restricted purge flow is enabled even when there is a small pressure difference across the mechanical valve. In addition, the higher engine airflow during the shallow manifold vacuum enables the higher purge flow rate to be better tolerated by the engine, without experiencing significant air-fuel excursions. The combination of the higher purge flow rate (and higher purge flow rate tolerance) during the shallow manifold vacuum conditions, and the frequent engine operation in the shallow manifold vacuum region allows the canister to be more thoroughly purged over a vehicle drive cycle. As such, this improves engine performance, fuel economy, and emissions compliance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.